All you need is two things.
1) Two camcorder batteries
2) Gaff tape
I can't think of a better tip for every person who shoots video or does photography. Simple and incredibly effective. This tip could really save the day in an event where your battery dies and you need to continue the shot as quickly as possible. Even fumbling around in your pockets to get a new battery could waste valuable time.
Here is the video I made as a demonstration.